At&t ASM learning 5

Command pointer

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Unconditional Branch:

Jump

Call

Interrupted

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1. Jump command

JMP = GOTO

Short jump

Near jump

Remote jump

2. Call commands

Call # Save the EIP value to the stack

RET # restore the original EIP value to the EIP register

3. Interrupt command

Int

Iret

Soft Interrupt 0x80

Hard interrupt

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Condition Branch:

Conditional jump command

Jxx address

Short jump

Near jump

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1. Use comparison commands

CMP operand1, operand2 # operand2-operand1

2. Use a flag

Use zero flag

Use overflow flag

Use parity flag

Use symbol flag

Use carry flag

Command for modifying carry flag

CLC # clear --> 0

CMC # Reverse Lookup

STC # settings --> 1

 

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Loop

Loop # loop until ECx = 0

Loope/loopz # loop until ECx = 0, or ZF = 0

Loopne/loopnz # loop until ECx = 0, or ZF = 1

Only short jump is allowed (that is, the 8-bit offset)

Count with ECx

When executing the loop command, first reduce ECx by 1 and then judge whether it is 0

Jcxz checks whether ECx is 0

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Salx # Left shift command

Leax # address transfer command

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Branch prediction:

The processor takes precedence over backward branches (that is, executing executed commands ),

If there is no backward branch, the command next to the jump command is preferred (that is, the command after the non-jump destination)

(For C-language if-else statements, the command next to if is preferred, instead of the command after else)

Branch Target Buffer: tracks each branch instruction executed by the processor, and then uses this data to predict the branch

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Optimization tips:

1. Remove Branch

Use cmov to replace the jump command

2. write code in the predictable branch first.

For example, the code in the loop is next to the code of the jump command.

3. Expand the loop